1. Field of the Invention
The present invention relates to Analog-to-Digital (A/D) converters.
2. Description of the Related Art
An A/D converter is designed according to specifications required for an electronic device in which the A/D converter is to be actually incorporated. If the electronic device has a plurality of sets of specifications required, the A/D converter is designed according to one that is strictest.
In general, as the characteristics of the A/D converter required are stricter, an operational amplifier, which is a component of the A/D converter, requires more currents. Therefore, when the A/D converter is used under at least two sets of specifications, a current more than necessary is supplied to the operational amplifier at the time of use with a less-strict set of specifications, resulting in wasteful power consumption in the A/D converter.
Specifically, for example, an A/D converter that switches between a low-speed operation and a high-speed operation lets a current more than necessary flow through the operational amplifier at the time of a low-speed operation. To solve this problem, for example, JP-A 2008-72742 (KOKAI) suggests that the current to be supplied to the operational amplifier is decreased at the time of a low-speed operation. In this patent document, however, a specific method of decreasing the supply current is not disclosed.
Also, in the design of the A/D converter, a margin under worst conditions is set in consideration of variations in characteristics of the operational amplifier for use, fluctuations in power-supply voltage, changes in environment temperature, and others. However, for example, the variations in characteristic of the operational amplifier for use may be more lenient in circuit characteristics. In this case, a current more than necessary flows through the operational amplifier, thereby also disadvantageously causing wasteful power consumption.